In general, di-TMP is obtained as a by-product upon production of trimethylolpropane (hereinafter referred to as “TMP”). Namely, when industrially producing TMP by aldol condensation and crossed Cannizzaro reaction between n-butyl aldehyde (hereinafter referred to as “NBD”) and formaldehyde in the presence of a base, di-TMP is produced as a by-product. The solution obtained by the reaction is extracted using a solvent after condensing or without condensing so as to a TMP extract (crude TMP). The crude TMP is purified by distillation under a high vacuum to obtain a TMP product together with a distillation still residue containing from 1 to 20% of TMP and from 20 to 50% of di-TMP. As a method of recovering di-TMP from the distillation still residue, there have been proposed distillation or crystallization using a solvent such as acetate (refer to Patent Document 1), and the like.
In these methods, it is required to separate and recover only di-TMP from a reaction mixture containing raw materials for TMP such as formaldehyde and NBD, a denatured material generated during the recovery of TMP by distillation, or by-products other than di-TMP such as an acetal of TMP and formaldehyde. Also, the content of di-TMP therein is low. Accordingly, it will be difficult to recover di-TMP with a high yield in an industrially efficient manner.
In order to solve these problems, studies have been made to increase an amount of di-TMP by-produced upon production of TMP, and there have been proposed a method of determining the reaction conditions to be specific (refer to Patent Document 2), a method of recovering 2-ethyl-2-propenal (hereinafter referred to as “ECR”) that is an intermediate reaction product and adding the same again to the reaction system (refer to Patent Document 3), and the like.
The amount of di-TMP produced in these methods is from about 10 to about 18 mol % based on NBD which is merely as low as from 2 to 4 times an amount of di-TMP by-produced in the conventional methods for producing TMP in which no measure for increasing an amount of di-TMP by-produced has been taken. The amount of di-TMP produced necessarily depends upon an amount of TMP produced because di-TMP is merely a by-product upon production of TMP. Thus, it is not adaptable for the increasing demand for di-TMP.
Also, there has been proposed the method of extracting di-TMP with an organic solvent having no affinity to water by adding the organic solvent to the reaction system (refer to Patent Document 4). However, this method, in which the aimed compound is contained in both of a water layer and an organic layer after the extraction, not only extremely complicates post-treatment after the reaction but also requires distillative recovery of the used organic solvent, and therefore, it is disadvantageous from the industrial viewpoints.
On the other hand, as a method of synthesizing di-TMP itself, there are known a method of producing di-TMP by subjecting two molecules of TMP to dehydrative condensation to produce di-TMP by an ether bond therebetween, a method of synthesizing di-TMP by reacting ECR with TMP and so on. In these methods, the amount of di-TMP produced is not limited by the amount of TMP produced unlike the methods of recovering di-TMP from the distillation still residue.
However, in the method of synthesizing di-TMP by subjecting two molecules of TMP to dehydrative condensation to produce an ether bond therebetween, because of the intermolecular reaction between TMP, the ether condensate that is necessarily generated between three or more molecules of TMP is inevitably by-produced. In order to suppress the by-production, the reaction rate of the dehydrative condensation reaction should be lowered, so that the amount of di-TMP produced is reduced, which is disadvantageous from the industrial viewpoints.
To improve these problems, Patent Document 5 discloses the method in which TMP having three alcoholic hydroxyl groups that are partially previously reacted and formed into a lower fatty acid ester is used as a raw material. However, even in this method, it is not possible to selectively react only two alcoholic hydroxyl groups in a molecule of TMP to form the lower fatty acid ester. Therefore, the method also fails to essentially solve the problem concerning production of the ether condensate of three or more molecules of TMP. In addition, this method further requires an additional step of regenerating di-TMP by hydrolyzing the di-TMP having esterified alcoholic hydroxyl groups which is produced from TMP having one or two esterified alcoholic hydroxyl groups. Thus, the method has increased economical burdens and therefore industrial disadvantages.
Further, in the method of producing di-TMP from ECR and TMP (refer to Patent Document 6), it is required that ECR as the raw material is separately synthesized. Since the yield of ECR from NBD is as low as about 40%, there occurs such a problem that a majority of NBD forms into an unnecessary by-product which should be discarded. Therefore, the method is problematic from the economical viewpoint.